could not understand one line i.e, handleServerResponse; (ajax tutorial)

The code that was given in video completely worked but I could not understand why it worked because it seemed to me that there was an error and I tried to correct it but found out that I was wrong but don't know why. The issue is in the file foodstore.js on the line

xmlHttp.onreadystatechange = handleServerResponse;

In that line, handleServerResponse is basically a function and when calling a function in javascript , shouldn't there be empty roundbrackets at the end ?? like this:

xmlHttp.onreadystatechange = handleServerResponse();

but when I added the brackets, the code stopped working. Why ??????????????